A conventionally proposed apparatus disclosed, for instance, in JP-A-2003-166967 exercises heater energization control so that the element resistance of a pump cell agrees with a target element resistance. When the element resistance of a sensor cell is outside a predefined range during heater energization control, this apparatus concludes that a sensor is deteriorated.    Patent Document 1:
JP-A-2003-166967    Patent Document 2:
JP-A-11-237363    Patent Document 3:
JP-A-2003-270194    Patent Document 4:
JP-A-2002-116180    Patent Document 5:
JP-A-2003-50227    Patent Document 6:
JP-A-2004-177179